How To Make Your Own DIY Seed Starting Mix (With Recipe!)

Seed starting mix can be expensive to buy, so I came up with my own recipe for homemade seed starting medium. This is the best seed starting mix, and it’s super easy to make too! In this post, I will share my recipe, and show you exactly how to make seed starter soil from scratch.

When I talk about starting seeds indoors, one of the first questions new gardeners ask me is about the best potting soil mix to use. This is a very important question, because the type of soil you use for growing seeds indoors really does make a huge difference – and it can make or break your seed starting success!

Using the wrong type of soil for planting seeds indoors is a common mistake. Many new gardeners think that “dirt is dirt”, so they either buy cheap potting soil, or try to use garden soil to start seeds indoors. This my friend is just a recipe for disaster.

Seed Starting Mix -vs- Potting Soil

The reason you can’t use cheap potting soil or garden soil to grow seeds indoors is because those types of soils will become compact in containers. When that happens, it’s extremely difficult (if not impossible) for seeds to germinate, and for the delicate seedling roots to grow.

Your seed starting medium should be porous so the soil stays light and fluffy, which makes it much easier for the seeds to germinate. A porous seedling mix also allows plenty of air around the roots – which is super important for healthy seedling growth.

In fact, the best potting soil to use for starting seeds indoors shouldn’t even contain soil at all.

What’s The Best Soil For Seed Germination?

The best seed starting medium to use for growing seeds indoors is a soilless seed starting mix that is both fast draining, and also holds moisture (sounds like a funny combo, I know).

I love making my own homemade seed starter mix, it’s super easy and it gives me the flexibility to modify the ingredients and make it just how I like it.

Plus I can make as much or as little as I need for starting my seeds, no need to have a big bag of seed starting mix lying around if I only need enough for one seedling tray.

Getting ready to make DIY seed starting mix

How To Make Seed Starting Mix

When I came up with my own soilless seed starting mix recipe, it was mainly because I had a bunch of the necessary ingredients laying around from making other potting soil recipes… and because buying pre-made seed starting mix is expensive.

But I also wanted to make sure the ingredients were easy for you to find too, so that I could share my recipe. These are all common ingredients that can be bought wherever you find potting soil for sale at your local garden center, or ordered online any time.

DIY Seed Starting Mix Ingredients

To make your own seed starting mix, you only need three main ingredients:

Starting seeds in coco coir on the other hand is very sustainable, it is the by-product of coconut processing. It’s a wonderful additive to any DIY soil mixes, and I really love the consistency of it. It’s my top choice for adding to my homemade seed starting mix!

Coir ingredient in seed starting soil mix recipe

Vermiculite

Vermiculite is a naturally occurring mineral, and is commonly found in potting soil mixes, especially soil for starting seeds. The reason vermiculite is commonly found in potting soil mixes is because it helps the soil retains water.

Seedlings won’t tolerate dried out soil, and vermiculite helps to keep the soil evenly moist so you won’t have to water as often.

Vermiculite DIY seed starting mix ingredient

Perlite

Perlite is a very lightweight soil amendment. It’s the white pieces that look like Styrofoam that you see in many commercial potting soils. It retains very little moisture, and prevents soil compaction. In other words, perlite helps the soil drain faster, and makes the mix porous (which is ideal for germinating seeds).

You can buy perlite in many garden centers or home improvement stores in the same section as you would find the potting soil for sale. A great alternative to perlite is pumice, just in case you have a hard time finding perlite in your local garden center.

Perlite ingredient in seed starting soil recipe

Supplies Needed:

Along with your seedling mix ingredients, you’ll need a few other supplies to help you measure and mix your everything. So grab these items before getting started…

What is a “part”? – This is a question that I get asked a lot when I talk about how to make potting soil. A “part” is just a generic unit of measure for portioning your ingredients. Use anything you want as your part, as long as you use the same thing for each “part”. For example if you use a 1 cup measure as your part, then this recipe would convert to 8 cups coir, 1 cup vermiculite, and 1 cup perlite.

How To Mix Your Homemade Seed Starting Soil

Making your own mix for starting seeds is easy. First, dump all the ingredients into a bucket or bowl…

Combine seedling mix ingredients

Then simply mix the ingredients with a spoon or trowel until they are well blended. Once the ingredients are blended together, you can fill your seedling trays and start planting seeds right away.

Mixing ingredients for DIY seed starting soil

That’s it. Told you it was easy to make your own seed starting mix. You could make a bunch ahead of time and store it for later use, or just mix up small batches as you need it.

I like to mix up a big batch, and then I store it in a plastic bucket in the garage so I always have seed starting mix on hand when I need it.

Storing Your Leftover DIY Seed Starter Mix

Whether you make your own seed starting mix, or choose to buy a commercial soil for starting seeds… make sure to store your leftover soil in an air tight container to avoid attracting fungus gnats. These air-tight seal lids work great to keep bugs out, and they fit on any standard five gallon bucket.

Store leftover seedling soil mix in sealed container

The best part about making your own soil for starting seeds is that you can experiment with different mixes. If you find that your seed trays are drying out too quickly, next time add more vermiculite to the mix. If your seed planting trays are staying too soggy, then add more perlite to your mix.

Oh, and this mix can also be used in seedling pots when it comes time for potting up your seedlings too!

Seedlings Growing In DIY Seed Starting Mix

If you want help to learn how to start your seeds indoors and get your seedlings off to a healthy start this year, my Starting Seeds Indoors eBook is for you! It’s a quick-start guide to help you get started growing the seeds for your garden.

Looking for even more help with growing your own seeds? Then you should enroll in the Seed Starting Course. This fun, in-depth course has everything you need to know about growing any plant you want from seed!

About Amy Andrychowicz

I live and garden in Minneapolis, MN (zone 4b). My green thumb comes from my parents, and I've been gardening most of my life. I’m a passionate gardener who loves growing everything from vegetables, herbs, and flowers to succulents, tropicals, and houseplants - you name, I've grown it! Read More...

This is great. It really does make a difference if you start seeds in the wrong kind of soil! I’ve used my own mix for both seed starting and container gardening for years and it works great. The vermiculite and perlite are especially helpful in maintaining healthy root systems!

When you say their not sterile, what do you mean? If they are still in the box, then they should be fine. But if they peat pellets/blocks already expanded or sitting in a really dirty location, then I would buy new ones.

Mold growth on top of the soil is caused by improper ventilation of your seedling trays, or too much moisture. I vent my seedling trays as soon as most of the seedlings have sprouted, and then remove the lids shortly after. I never spray anything on my seedlings, giving them proper ventilation always does the trick. Here’s a post I wrote about mold on seedlings, and here are even more tips that might help you… Tips For Starting Seeds Indoors

Coffee grounds are good for the soil out in the garden, and to add to your compost bin… but I would not use it in a seed starting mix (or any other indoor potting mix). Coffee grounds mold really quickly, and you don’t want to encourage mold growth in your seed trays.

Just wanted to check, but after the seeds have sprouted and you transplant them into a larger container, you are recommending that you use the same seed starting mixture? Do you have to add some plant food to the mixture?

Yes, I use this same seed starting mix when I put up my seedlings. Those tiny seedling roots need a light and porous mix so they can easily grow (regular potting soil is too heavy for seedlings). I don’t add plant food to the soil at this point, I feed my seedlings with liquid fertilizer until they get transplanted into the garden or their final summer containers. Here’s more info about growing seeds indoors, including details about fertilizing seedlings.